1. Field of the Invention
The present invention relates to a projection exposure apparatus used in the production of extremely fine patterns such as semiconductor integrated circuits.
2. Description of the Prior Art
With the recent trend toward increasing the level of integration of semiconductor integrated circuits, projection exposure apparatus heretofore used for projecting and transcribing the pattern of a reticle onto a wafer have predominatly consisted of so-called steppers so designed that a high-resolution reduction projection lens is mounted and a wafer is moved in a step and repeat manner, thereby projecting and transcribing patterns sequentially on the plurality of exposure areas of the wafer.
The stepper is required to meet a degree of accuracy of alignment (registration of the projection image of a reticle with the patterns formed on a wafer by the preceding wafer process) corresponding to the resolution of the projection lens. Generally, an alignment accuracy of 1/5 to 1/10 of the minimum resolution linewidth of the projection lens is required. Therefore, an alignment accuracy of 0.08 to 0.16 .mu.m is required for a lens which for example resolves patterns of 0.8 .mu.m width for VLSI manufacturing purposes.
In order to realize such a highly accurate alignment, it is important to adjust the various parts of the apparatus in temperature and the usual practice in the past has been such that as shown in FIG. 4, the whole apparatus is arranged inside a temperature adjusting chamber 120 and the internal air temperature of the chamber 120 is controlled. In FIG. 4, a reticle 102 is illuminated by an illuminating optical system 101 so that the image of the pattern formed on the reticle 102 is projected and transcribed through a projection lens 103 on an exposure area of a wafer 104 which is positioned just below the projection lens 103.
The wafer 104 is movable by a Z-stage 105 in a vertical direction (the direction of the optical axis of the projecting lens) and it is also movable in X and Y directions (the directions perpendicular to the optical axis of the projection lens) by an X-stage 106 and a Y-stage 107, respectively. Then, the X-direction and Y-direction positions of the wafer 104 are monitored by means of a laser interferometer-type linear scale 110 and the wafer 104 is positioned in a desired position by the X-stage 106 and the Y-stage 107. Also, the height of the surface of the wafer 104 is detected by a height sensor which is not shown and its height is adjusted by the Z-stage 105.
This apparatus on the whole is mounted on a surface table 108 subjected to vibration damping by a plurality of vibration damping bases 109 and in this condition the apparatus is installed within the temperature adjusting chamber 120. The air adjusted to a preset temperature by a heat exchanger 121 is blown into the temperature adjusting chamber 120 through a blowing duct 122 and the blown air is discharged through a return duct 123. Although not shown in the Figure, the air delivered from the blowing duct 122 flows into the surroundings of the reticle 102 as well as the surroundings of the projection lens 103 and the wafer stages (105, 106 and 107).
In the case of the conventional technique described above, however, the whole apparatus is enclosed by the partition wall of the single chamber thereby air conditioning the apparatus as a whole and therefore the apparatus is affected considerably by the heat generated from such heat sources as various electronic component parts, motor and laser, thereby making it difficult to maintain the temporal and spatial uniformity of the refractive index of the air in the space containing the projection optical system between the reticle and the wafer. In other words, the refractive index of the air varies in dependence on the temperature and thus the image forming position is varied or the image is distorted, thereby giving rise to a major cause of deteriorating the alignment accuracy.